Various methods are known for the formation of a plasma or ionized gas. Frequently a plasma is employed for chemical reaction or in connection with chemical analysis, and for such purposes the most often used methods of plasma generation are direct conduction of electric charge into an ionizable gas or inductive coupling by means of an appropriate magnetic field. Also sometimes employed is microwave energization by means of an appropriate wave energy in a resonant cavity. In all of these, a gas discharge is energized, causing ionization of the gas, the most frequently employed gas for this purpose being argon or other so-called inert gas.
One valuable use and application of plasma generators in recent years has been for spectrographic or spectrometric analysis of trace material in a sample, and the present invention while not limited to this purpose is aimed at such use. For this application of plasma, a small sample of a material to be tested is brought into the near vicinity, or within the field of energy, of a plasma jet, causing the sample to become energized to emit identifiably characteristic radiation. By means of a spectrometer or other radiation-identifying means this radiation is identified and measured, thus identifying and quantifying the nature of the element or other material in the sample. For this purpose it is important to have a small sized plasma jet so that as small a sample as possible can be analyzed and to have the reaction or analyzing area small, compact, and uniformly reproducible. It is also important to keep the plasma device free from contamination, particularly to keep it free from contaminants which may remain in the equipment for protracted periods of time.
One plasma jet device which has found commercial use for such spectrometric analysis is that of Elliott et al U.S. Pat. No. 4,009,413. According to that patent, a plasma is formed in the shape of an inverted V by means of anode and cathode electrodes which direct jets of argon or similar gas to form a plasma having a bend or elbow. A sample to be analyzed is fed into the pocket of the elbow where it is energized by the plasma and is thus caused to emit its characteristic radiation.
One difficulty with this type of device, believed to be inherent in its anode-cathode nature, is that the electrodes erode, thus requiring frequent replacement and/or adjustment and, in addition, running the risk of introducing contamination in the form of the anode or cathode material.